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Ecological Applications

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Flow regulation and fragmentation of the world's rivers threaten the integrity offreshwater ecosystems and have resulted in the loss or decline of numerous fish species.Pelagic-spawning fishes (pelagophils) are thought to be particularly susceptible to river regulationbecause their early life stages (ichthyoplankton) drift until becoming free-swimming, although theextent of transport is largely unknown. Transport velocity and distance were determined forpassively drifting particles, which mimicked physical properties of ichthyoplankton, in two large,regulated rivers (Rio Grande and Pecos River) of the arid Southwest United States. Particle driftdata were incorporated into celerity-discharge equations (r2 > 0.90; P < 0.001), and reachspecifictransport velocity was modeled as a function of discharge. Transport velocities ofparticles exceeded 0.7 m/s in all river reaches during typical spawning flows (i.e., reservoirreleases or rainstorms) and were greatest in highly incised and narrow channel reaches. Meantransport distance of particles released in the Pecos River during sustained reservoir flows (141.1km; 95% CI = 117.0-177.5 km) was significantly longer than during declining reservoir flows thatmimicked a natural rainstorm (52.4 km; 95% CI = 48.8-56.5 km). Mean transport distance ofparticles in the Rio Grande during sustained reservoir flows was 138.7 km (95% CI = 131.0-147.2km). There are 68 dams and 13 reservoirs that fragment habitats and regulate flow in the RioGrande Basin (Rio Grande and Pecos River) in areas historically occupied by pelagophils. Whilethe basin historically provided 4029 km of free-flowing riverine habitat, reservoir habitat nowrepresents > 10% of the longitudinal distance. Only five unfragmented nonreservoir reaches >100 km remain in the Rio Grande, and two remain in the Pecos River. Pelagophils wereextirpated from all reservoirs and from nearly all short, fragmented reaches (< 100 km) of the RioGrande Basin, but at least some fraction persisted in all longer reaches (> 100 km). The recoveryand long-term persistence of pelagophils in regulated rivers, including those in this study, willlikely depend on reestablishment and protection of long unfragmented reaches coupled withmimicry of the natural flow regime. http://dx.doi.org/10.1890/06-1252.1